1. Field of the Invention
The present invention relates to a control unit for a clutchless, variable displacement compressor used in air conditioning systems for vehicles and operated in response to an external signal.
2. Description of Related Art
In known variable displacement compressors used in conventional air conditioning systems for vehicles, a temperature of the compressor, such as a temperature of a housing, may be elevated by operation of the compressor to an excessive level. When detected by a thermistor, such an excessive increase of the temperature of the compressor may be prevented, for example, as described in Japanese Patent Application No. JP-A-11-159449.
In such known systems, a degree of protection of the compressor may be achieved when the temperature of the compressor increases while the air conditioning system is in operation. Nevertheless, such clutchless, variable displacement compressors are generally driven integrally with an engine. Thus, the compressors are rotated at a condition of minimum displacement of discharge together with the engine, even when the air conditioning system is not in operation, and such known systems are unable to protect the compressors from damage. In particular, because little or no refrigerant is circulated in this condition of minimum displacement of discharge, lubrication in the compressor may be insufficient. If the rotational speed is increased markedly under such conditions, the temperature of the compressor may increase excessively. If such a condition persists, the reliability of the compressor may diminish, and the compressor may be damaged.
A need has arisen for preventing a persistent increase in the temperature of the compressor. Accordingly, it is an object of the present invention to provide a control unit for a variable displacement compressor, particularly for a clutchless, variable displacement compressor, which protects the compressor against a persistent increase in the temperature of the compressor by detecting an increase of the temperature of the compressor, when the compressor is driven at its minimum displacement. Such a control unit does not require a temperature sensor such as a thermistor, yet prevents the occurrence of an excessive increase of temperature.
To achieve the foregoing and other objects, a control unit for a variable displacement compressor according to the present invention is provided. The control unit for a clutchless variable displacement compressor is used in an air conditioning system for vehicles, and the compressor is equipped with a solenoid. The discharge displacement of the compressor is controlled by varying a pressure in the compressor by a control valve adjusted in response to a current flowing in the solenoid. The control unit comprises solenoid drive means which comprises switching elements for adjusting the current flowing in the solenoid; voltage detection means, e.g., a voltmeter, for detecting a voltage applied to the solenoid; current detection means for detecting the current flowing in the solenoid; and control means for delivering current to the solenoid drive means during a first predetermined time period in which the air conditioning system for vehicles is not in operation and during which current has been prevented from flowing to the solenoid continuously during a second predetermined time period. A resistance of a coil of the solenoid is calculated from a voltage detected by the voltage detection means and a current detected by the current detection means. A temperature of the compressor is determined from the calculated resistance based on a predetermined relationship between the resistance and the temperature of the coil of the solenoid. The air conditioning system for vehicles begins operation and operation of the variable displacement compressor are terminated by supplying current to the solenoid when the determined temperature exceeds a set value.
The control unit for a variable displacement compressor may be structured as described below. Specifically, the control unit may comprise rotational speed detection means, e.g., a tachometer, for detecting a physical value corresponding to a rotational speed of the compressor. In the control unit, the control means controls the supply of current to the solenoid drive means during the first predetermined time period in which the air conditioning system for vehicles is not in operation. Current has been prevented from flowing to the solenoid during the second predetermined time period, and an output value of the rotational speed detection means exceeds a predetermined value continuously over a third predetermined time period. The resistance of the coil of the solenoid is calculated from a voltage detected by the voltage detection means and a current detected by the current detection means, such as by a voltmeter in combination with an ammeter or by a voltammeter. A temperature of the compressor is determined from the calculated resistance based on the predetermined relationship between the resistance and the temperature of the coil of the solenoid. When the air conditioning system for the vehicle begins operation, the variable displacement compressor operates to supply current to the solenoid when the calculated temperature exceeds the set value.
In another embodiment, the control unit for a variable displacement compressor may be structured as described below. Specifically, the control unit may comprise thermal load detection means for detecting a thermal load, such as an outside air temperature, and in the control unit, the control means controls the supply of current flowing to the solenoid drive means during the first predetermined time period, when the air conditioning system for vehicles is not in operation. Current has been prevented from flowing to the solenoid during the second predetermined time period, and an output value of the thermal load detection means exceeds a predetermined value continuously during a fourth predetermined time period. The a resistance of the coil of the solenoid is calculated from a voltage detected by the voltage detection means and a current detected by the current detection means. A temperature of the compressor is determined from the calculated resistance based on the predetermined relationship between the resistance and the temperature of the coil of the solenoid. When the air conditioning system for vehicles begins operation, the variable displacement compressor operates to supply current to the solenoid when the calculated temperature exceeds the set value.
In still another embodiment, the embodiment control unit for a variable displacement compressor may be structured as described below. Specifically, the control unit may comprise thermal load detection means for detecting a thermal load, such as an outside air temperature, and rotational speed detection means for detecting a physical value corresponding to a rotational speed of the compressor. In the control unit, the control means controls supply of current flowing to the solenoid drive means during the first predetermined time period, when the air conditioning system for vehicles is not in operation. Current has been prevented from flowing to the solenoid during the second predetermined time period, and output values for each of the thermal load detection means and the rotational speed detection means have exceeded respective predetermined values continuously during a fifth predetermined time period. The resistance of the coil of the solenoid is calculated from a voltage detected by the voltage detection means and a current detected by the current detection means at that time. A temperature of the compressor is determined from the calculated resistance based on the predetermined relationship between a resistance and a temperature of the coil of the solenoid. When the air conditioning system for vehicles begins operation, the variable displacement compressor operates to supply current to the solenoid when the calculated temperature exceeds the set value.
Moreover, the above-described control means may be constructed so as to cease operation of the air conditioning system for vehicles after a sixth predetermined time period.
In such a control unit for a variable displacement compressor according to the present invention, when the air conditioning system for vehicles is not in operation and the variable displacement compressor has been driven during the second predetermined time period in a minimum displacement condition (i.e., without current being supplied to the solenoid). The solenoid drive means intentionally is provided with current during the first predetermined time and the solenoid is supplied with current. At that time, because the voltage applied across the solenoid and the current flowing in the solenoid are detected by the voltage detection means and the current detection means. From these detected voltage and current values, the resistance value of the coil of the solenoid at a particular time may be calculated. By predetermining a relationship between a resistance and a temperature of the coil of the solenoid in advance the temperature of the compressor may be determined from the calculated resistance based on the predetermined relationship. When the calculated temperature of the compressor exceeds a preset value, the air conditioning system for vehicles may begin operation, and the solenoid is supplied with current. The variable displacement compressor is operated, and refrigerant is circulated intentionally, whereby the compressor is cooled and the inside of the compressor is lubricated. Consequently, the increase of the temperature of the compressor may be suppressed, a smooth operation of the compressor may be ensured by the sufficient lubrication, the occurrence of an abnormal operating conditions may be prevented, and the compressor may be protected. Operation of the air conditioning system then may again cease after the reduction of the compressor temperature and the lubrication of the compressor interior.
Thus, in the control unit for a clutchless, variable displacement compressor according to the present invention, the temperature of the variable displacement compressor may be properly estimated when the air conditioning system is not in operation. This may be accomplished without the use of a temperature sensor. When the air conditioning system is not in operation for a predetermined period of time and the temperature of the compressor rises to an excessively high temperature, the state of the air conditioning system is turned to be in operation, and the compressor is cooled and any lack of lubrication is resolved. Thus, damage to the compressor may be prevented or reduced, and the compressor may be appropriately protected.
Further, because the detection of the voltage across and of the current flowing through the solenoid may be restricted to occur under a limited set of conditions (e.g., an outside air temperature or a rotational speed of the compressor or an outside air temperature plus a rotational speed of the compressor), the normal operation of the air conditioning system may not be much disturbed. Moreover, because the time for operating the air conditioning system is limited, the operation of the air conditioning system also may not be much disturbed.
In addition, when determining factors, such as engine rotational speed and outside air temperature, are factored into the control of the compressor, the temperature of the compressor may be estimated more accurately.